This invention relates to an amplifier circuit and, more particularly, to an amplifier circuit with a small input current which usually involves a noise problem in low frequencies of the input signal.
Examples of this type of ordinary amplifier circuit are shown in FIGS. 1 and 2. In the amplifier circuit shown in FIG. 1, N channel MOS transistors M1 and M2 form a differential amplifier. V1 and V2 designate power sources with differential terminal voltages +Vin/2 and -Vin/2, respectively. 10 designates a bias power source for the power sources V1 and V2. An input signal S1 at a differential potential Vin is applied between the gates G1 and G2 of the transistors G1 and G2. Reference numeral 11 designates a current source, and Z1 and Z2 denote loads. Numeral 12 is representative of a power line connected to a high potential Vcc. GND denotes ground. The output voltage Vout of this circuit is derived from the drains of the MOS transistors M1 and M2.
In the circuit shown in FIG. 2, PNP bipolar transistors Q1 and Q2 form a differential amplifier. An input signal S1 is applied between the bases B1 and B2 of these transistors Q1 and Q2. Emitters of the transistors Q1 and Q2 are connected in series to transistors Q3 and Q4, respectively. The output voltage Vout is derived from the collectors of the transistors Q3 and Q4. The bias source 13 is provided for the transistors Q3 and Q4.
In the FIG. 1 circuit, the N channel MOS transistors M1 and M2 are used as input elements. In this type of circuit, the noise power, i.e., 1/f noise, is increased in low frequencies of the input signal. This noise originates from the physical nature of the input elements per se. The N channel MOS transistor causes approximately three times the noise that the P channel transistor causes. Therefore, the FIG. 1 circuit of prior art using the N channel transistor is inappropriate for use in the amplifier operating in audio frequencies.
In the FIG. 2 circuit, the PNP bipolar transistors Q1 and Q2 are used as the input elements. In this type of transistor, the base current flow inevitably and adversely influences the input signal, particularly when the input current is small. The pair of PNP transistors Q1 and Q2, and the pair of NPN transistors Q3 and Q4 are each connected in a complementary fashion. However, it is very difficult to manufacture the transistors of the different polarities with uniform performances.